1. Field of the Invention.
The present invention relates generally to fluid mass flow sensing devices, and more particularly to a fluid flow sensor which utilizes heat transfer to the fluid as a measure of flow. The sensor utilizes a hot-wire flow sensing technique and may be employed as part of an autonomous controller for regulating the fluid flow. Several such sensors may be deployed and operate essentially independent of one another in some applications.
2. Description of the Related Art.
Aircraft require the accurate measurement and control of airflow for performance and environmental functions such as engine starting, equipment cooling, and environmental control for cockpit and cabin compartments. These functions are typically managed by the aircraft bleed air control and environmental control system which is an integrated system of components that monitor the conditions and control the state of these and other functions throughout the aircraft. Traditional designs attempt to embed the electronic controls within a centralized hub controller and execute sensor readback/actuator drive output over lengthy wire bundles to respective sensors and actuators. The flow measurement in these functions is typically achieved through the use of mass flow sensors such as described by Novak et al. in U.S. Pat. No. 3,927,567. This patent discloses a hot-wire mass air flow meter for duct mounted purposes. The environmental system controller uses the output of the sensor as an input to control algorithms to perform system control functions.
It is desirable from both a cost and weight standpoint to eliminate the lengthy wire bundles and provide instead a sensor which is capable of performing stand-alone control functions autonomous of the overall system controller. To perform these autonomous control functions it is desirable for the sensor to have a current output scaled to the input of the controlled device. It is desirable to also provide a sensor that may be easily incorporated into existing fluid systems.
The present invention provides solutions to the above problems by consolidating sensor feedback, input /output signal filtering, and output actuator control into a single entity. All that is required for operation is power. Raw data (in this case, flow) is sensed at the duct and translated into an appropriate output signal which can be used as a control signal. The entity senses flow in a duct by monitoring the heater current required to maintain a programmed delta temperature relationship between a heated and an ambient element. The measured heater current (which is a nonlinear function of flow) is then translated into an output signal which is representative of airflow.
The invention comprises, in one form thereof, a flow sensor and controller assembly having a housing and a cantilevered flow sensing probe anchored to the housing at one end, and extending into the flow in good heat transfer relation therewith. There is an electrically heatable member within the probe and a first temperature sensitive resistance within the probe in close proximity to the heatable member. A second temperature sensitive resistance is located within the probe remote from the electrically heatable member so as to be thermally isolated therefrom. Electrical circuitry within the housing supplies substantially constant current to each of the temperature sensitive resistances and amplifies the signals for processing. These amplified signals are then processed along with a delta temperature set point signal to provide an input to the heater current drive circuitry, which generates the current to power the heated member. The heater current used to perform this function is then measured and processed to provide control signals related to the flow rate, or to influence the flow rate, or to affect characteristics of the fluid flow.